Chapter 11 Liquids, Solids, and Intermolecular Forces

1. Chapter 11Liquids, Solids, and Intermolecular Forces

2. Climbing Geckos • Geckos can adhere to almost any surface • Recent studies indicate that this amazing ability is related to intermolecular attractive forces • Geckos have millions of tiny hairs on their feet that branch out and flatten out on the end • setae = hairs, spatulae = flat ends • This structure allows the gecko to have unusually close contact to the surface – allowing the intermolecular attractive forces to act strongly

3. Properties of the three Phases of Matter • Fixed = keeps shape when placed in a container • Indefinite = takes the shape of the container

4. Three Phases of Water Notice that the densities of ice and liquid water are much larger than the density of steam Notice that the densities and molar volumes of ice and liquid water are much closer to each other than to steam Notice that the densities of ice is larger than the density of liquid water. This is not the norm, but is vital to the development of life as we know it.

5. Degrees of Freedom • Particles may have one or several types of freedom of motion • and various degrees of each type • Translational freedom is the ability to move from one position in space to another • Rotational freedom is the ability to reorient the particles direction in space • Vibrational freedom is the ability to oscillate about a particular point in space

6. Solids • Particles packed close and are fixed • though they may vibrate • Incompressible • Retention of shape and volume • Do not flow • Crystalline Solids - particles arranged in an orderly geometric pattern • salt and diamonds • Amorphous Solids – particles do not show a regular geometric pattern over a long range plastic and glass

7. Liquids • Particles are closely packed, but they have some ability to move around • Incompressible • Take the shape of their container and to flow • Do not have enough freedom to escape or expand to fill the container

8. Gases • Particles have complete freedom of motion and are not held together • Particles in constant motion, bumping into each other and the container • There is a large amount of space between the particles • compared to the size of the particles • molar volume of the gas state of a material is much larger than the molar volume of the solid or liquid states • Compressible • Expand to fill and take the shape of their container, and will flow

9. Compressibility

10. Kinetic – Molecular Theory • What state a material is in depends largely on two major factors 1. the amount of kinetic energy the particles possess 2. the strength of attraction between the particles • These two factors are in competition with each other

11. States and Degrees of Freedom • The molecules in a gas have complete freedom of motion • their kinetic energy overcomes the attractive forces between the molecules • The molecules in a solid are locked in place, they cannot move around • though they do vibrate, they don’t have enough kinetic energy to overcome the attractive forces • The molecules in a liquid have limited freedom – they can move around a little within the structure of the liquid • they have enough kinetic energy to overcome some of the attractive forces, but not enough to escape each other

12. Kinetic Energy • Increasing kinetic energy increases the motion energy of the particles • The more motion energy the molecules have, the more freedom they can have • The average kinetic energy is directly proportional to the temperature • KEavg = 1.5 kT

13. Attractive Forces • The particles are attracted to each other by electrostatic forces • The strength of the attractive forces varies, some are small and some are large • The strength of the attractive forces depends on the kind(s) of particles • The stronger the attractive forces between the particles, the more they resist moving • though no material completely lacks particle motion

14. Kinetic–Molecular Theory of Gases • When the kinetic energy is so large it overcomes the attractions between particles, the material will be a gas • In an ideal gas, the particles have complete freedom of motion – especially translational • This allows gas particles to expand to fill their container • gases flow • It also leads to there being large spaces between the particles • therefore low density and compressibility

15. Gas Structure Gas molecules are rapidly moving in random straight lines, and are free from sticking to each other.

16. Kinetic–Molecular Theory of Solids • Solids exhibit strong attractive forces limiting the kinetic energy of the particles • Solid packing results in no translational or rotational motion • the only freedom they have is vibrational motion

17. Kinetic–Molecular Theory of Liquids • Liquids exhibit attractive forces strong enough to partially overcome the kinetic energy • Particles are packed together with only very limited translational or rotational freedom

18. Explaining the Properties of Liquids • Because the particles are in contact… • Higher densities than gases • Incompressible • Because the particles have limited translational freedom… • Indefinite shape allowing for flow • Take the shape of the container • Because the limit on their freedom keeps the particles from escaping each other • Liquids have a definite volume

19. Phase Changes • The attractive forces between the molecules are fixed changing the material’s state requires changing the amount of kinetic energy that is limiting the particles freedom • Solids melt when heated because the particles gain enough kinetic energy to partially overcome the attractive forces • Liquids boil when heated because the particles gain enough kinetic energy to completely overcome the attractive forces • the stronger the attractive forces, the higher you will need to raise the temperature • Gases can be condensed by decreasing the temperature and/or increasing the pressure • pressure can be increased by decreasing the gas volume • reducing the volume reduces the amount of translational freedom the particles have

20. Phase Changes

21. Intermolecular Attractions • The strength of the attractions between the particles of a substance determines its state • At room temperature, moderate to strong attractive forces result in materials being solids or liquids • The stronger the attractive forces are, the higher will be the boiling point of the liquid and the melting point of the solid • other factors also influence the melting point

22. Why Are Molecules Attracted to Each Other? • Intermolecular attractions are due to attractive forces between opposite charges • + ion to − ion • + end of polar molecule to − end of polar molecule • H-bonding especially strong • even nonpolar molecules will have temporary charges • Larger charge = stronger attraction • Longer distance = weaker attraction • However, these attractive forces are small relative to the bonding forces between atoms • generally smaller charges • generally over much larger distances

23. Trends in the Strength of Intermolecular Attraction • The stronger the attractions between the atoms or molecules, the more energy it will take to separate them • Boiling a liquid requires we add enough energy to overcome all the attractions between the particles • However, not breaking the covalent bonds • The higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces

24. Kinds of Attractive Forces • Dispersion Forces - Unequal electron distribution leads to attractions causing temporary polarity in the molecules • Dipole–Dipole Attractions - Permanent polarity in the molecules due to their structure leads to attractive forces • Hydrogen Bonds - An especially strong dipole–dipole attraction resulting when H is attached to an extremely electronegative atom.

25. Dispersion Forces • Fluctuations in the electron distribution in atoms and molecules result in a temporary dipole • region with excess electron density has partial (─) charge • region with depleted electron density has partial (+) charge • The attractive forces caused by these temporary dipoles are called dispersion forces • aka London Forces • All molecules and atoms will have them • As a temporary dipole is established in one molecule, it induces a dipole in all the surrounding molecules

26. Dispersion Force

27. + + + + + + + + + + + + + + + + - - - + + + + - - + + + + + + - - - - − − − − - − − − − − − − - - − − Size of the Induced Dipole • The magnitude of the induced dipole depends on several factors • Polarizability of the electrons • volume of the electron cloud • larger molar mass = more electrons = larger electron cloud = increased polarizability = stronger attractions • Shape of the molecule • more surface-to-surface contact = larger induced dipole = stronger attraction molecules that are flat have more surface interaction than spherical ones larger molecules have more electrons, leading to increased polarizability

28. Effect of Molecular Sizeon Size of Dispersion Force As the molar mass increases, the number of electrons increases. Therefore the strength of the dispersion forces increases. The Noble gases are all nonpolar atomic elements The stronger the attractive forces between the molecules, the higher the boiling point will be.

30. Properties of Straight Chain AlkanesNonPolar Molecules

31. Boiling Points of n-Alkanes

33. Effect of Molecular Shapeon Size of Dispersion Force the larger surface-to- surface contact between molecules in n-pentane results in stronger dispersion force attractions

34. Alkane Boiling Points • Branched chains have lower BPs than straight chains • The straight chain isomers have more surface-to-surface contact

35. Practice – Choose the Substance in Each Pair with the Higher Boiling Point a) CH4 C4H10 b) C6H12 C6H12

36. Practice – Choose the Substance in Each Pair with the Higher Boiling Point a) CH4 CH3CH2CH2CH3 b) CH3CH2CH=CHCH2CH3 cyclohexane Both molecules are nonpolar larger molar mass Both molecules are nonpolar, but the flatter ring molecule has larger surface-to-surface contact

37. Dipole–Dipole Attractions • Polar molecules have a permanent dipole • because of bond polarity and shape • dipole moment • as well as the always present induced dipole • The permanent dipole adds to the attractive forces between the molecules • raising the boiling and melting points relative to nonpolar molecules of similar size and shape

38. Effect of Dipole–Dipole Attraction on Boiling and Melting Points

39. replace with the figure 11.8

40. Formula Lewis Structure Bond Polarity Molecule Polarity Example 11.1b: Determine if dipole–dipole attractions occur between CH2Cl2 molecules Given: Find: CH2Cl2, EN C = 2.5, H = 2.1, Cl = 3.0 If there are dipole–dipole attractions Conceptual Plan: Relationships: EN Difference Shape molecules that have dipole–dipole attractions must be polar Cl—C 3.0−2.5 = 0.5 polar Solution: polar bonds and tetrahedral shape = polar molecule 4 bonding areas no lone pairs = tetrahedral shape C—H 2.5−2.1 = 0.4 nonpolar polar molecule; therefore dipole–dipole attractions

41. or Practice – Choose the substance in each pair with the higher boiling point a) CH2FCH2F CH3CHF2 b)

42. or Practice – Choose the substance in each pair with the higher boiling point a) CH2FCH2F CH3CHF2 more polar b) polar nonpolar

43. Hydrogen Bonding • When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it • O─H, N─H, or F─H • Because hydrogen has no other electrons, when its electron is pulled away, the nucleus becomes deshielded • exposing the H proton • The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules

44. H-Bonding HF

45. H-Bonding in Water

46. H-Bonds • Hydrogen bonds are very strong intermolecular attractive forces • stronger than dipole–dipole or dispersion forces • Substances that can hydrogen bond will have higher boiling points and melting points than similar substances that cannot • But hydrogen bonds are not nearly as strong as chemical bonds • 2 to 5% the strength of covalent bonds

47. Effect of H-Bonding on Boiling Point

48. For nonpolar molecules, such as the hydrides of Group 4, the intermolecular attractions are due to dispersion forces. Therefore they increase down the column, causing the boiling point to increase. HF, H2O, and NH3 have unusually strong dipole-dipole attractions, called hydrogen bonds. Therefore they have higher boiling points than you would expect from the general trends. Polar molecules, such as the hydrides of Groups 5–7, have both dispersion forces and dipole–dipole attractions. Therefore they have higher boiling points than the corresponding Group 4 molecules.

49. Example 11.2: Which of these compounds is a liquid at room temperature (the others are gases). Why? MM = 30.03 Polar No H-Bonds MM = 34.03 Polar No H-Bonds MM = 34.02 Polar H-Bonds Step 3. Evaluate the strengths of the total intermolecular attractive forces. The substance with the strongest will be the liquid. Step 1. Determine the kinds of intermolecular attractive forces Step 2. Compare intermolecular attractive forces Because the molar masses are similar, the size of the dispersion force attractions should be similar Formaldehyde: dispersion forces: MM 30.03, trigonal planar dipole–dipole: very polar C=O bond uncancelled H-bonding: no O–H, N–H, or F–Htherefore no H-bond Fluoromethane: dispersion forces: MM 34.03, tetrahedral dipole–dipole: very polar C–F bond uncancelled H-bonding: no O–H, N–H, or F–H therefore no H-bond Hydrogen peroxide: dispersion forces: MM 34.02, tetrahedral bent dipole–dipole: polar O–H bonds uncancelled H-bonding: O–H, therefore H-bond Because only hydrogen peroxide has the additional very strong H-bond additional attractions, its intermolecular attractions will be the strongest. We therefore expect hydrogen peroxide to be the liquid. Because all the molecules are polar, the size of the dipole–dipole attractions should be similar Only the hydrogen peroxide also has additional hydrogen bond attractions

50. Practice – Choose the substance in each pair that is a liquid at room temperature (the other is a gas) a) CH3OH CH3CHF2 b) CH3-O-CH2CH3 CH3CH2CH2NH2 can H-bond can H-bond